Review of Medical Physiology - William F. Ganong 2002

Cardiovascular System
Regional Circulation
Cutaneous Circulation

The amount of heat lost by the body is largely regulated by varying Blood flow through the Skin (see Chapter 14). The fingers, toes, palms, and earlobes contain well-innervated anastomoses between arterioles and venules (arteriovenous anastomoses; see Chapter 30). BLOOD FLOW IN response to thermoregulatory stimuli can range from 1 to 150 mL/100 g of skin per min. These changes are believed to occur because blood is shunted through the anastomoses. The subdermal capillaries and venous plexus serve as an important blood reservoir, and the skin is one of the few sites where vascular responses can be observed visually.

White Dermographism

If the skin is stroked lightly with a pointed object, the resulting lines appear pale (white dermographism). The mechanical stimulus causes the precapillary sphincters to contract, forcing blood out of the capillaries and small venules. The response appears within 15 s.

Triple Response

If the skin is stroked more firmly with a pointed object, a red area (red dermographism) appears within about 10 s instead of white dermographism. After a few minutes, signs of edema and mottled redness can be observed around the injury site. The rapid reddening is due to capillary dilation—a direct capillary response to pressure. The Swelling (wheal) is a local edema caused by increased permeability of capillaries and postcapillary venules, followed by fluid extravasation. The flare extending from the injury site (arterial hyperemia) results from arteriolar dilation. This three-component response is called the triple response; it is part of the normal response to injury (see Chapter 20) and persists following complete sympathectomy. Arterial hyperemia is absent during local anesthesia and in denervated skin following sensory nerve degeneration. However, it is observed immediately after a block of a nerve or its branch proximal to the injury site. These observations, along with other data, confirm that this reaction is mediated by an axon reflex—a response in which impulses generated after injury in sensory nerves travel antidromically (in the opposite direction along other branches of sensory nerve fibers) (Fig. 32-17). This is the only situation in the body where a physiological effect is produced by antidromic conduction. The increase in vascular permeability during edema formation is partly related to histamine or histamine-like compounds released from local mast Cells and acting via H₁ receptors. The transmitter released at the central terminals of sensory C-fiber Neurons is substance P (see Chapter 4). Substance P and CGRP are present throughout all regions of these neurons. Both compounds dilate arterioles, and substance P also causes fluid extravasation. Effective non-peptide substance P antagonists have now been developed that reduce fluid leakage from vessels. Thus, both peptidergic and histaminergic components can be distinguished in The Development of edema.

Reactive Hyperemia

The vascular response occurring in many Organs, yet visible in the skin, known as reactive hyperemia, is the increase in blood flow within an area where Circulation is restored following a period of occlusion. If the Blood supply to the limbs is disrupted, the skin arterioles below the occlusion site dilate. Upon restoration of blood flow, the influx of blood into the dilated vessels turns the skin fiery red. Because oxygen diffuses only a short distance through the skin, reactive hyperemia does not develop if limb blood flow is arrested in a 100% oxygen atmosphere. Thus, arteriolar dilation is related to the local effects of Hypoxia.

Class="center">

Fig. 32-17. Axon reflex.

Generalized Responses

Noradrenergic nerve stimulation, along with circulating epinephrine and norepinephrine, causes constriction of cutaneous Blood Vessels. Vasodilator nerves to cutaneous vessels have not been identified, and vasodilation occurs through a decrease in vasoconstrictor tone, as well as The production of bradykinin in Sweat Glands and vasodilator metabolites. Skin color and Temperature also depend on the state of the capillaries and venules. In cold, blue, or gray skin, arterioles constrict while capillaries dilate; in warm skin, both arterioles and capillaries dilate.

Because pain stimuli elicit diffuse firing in noradrenergic nerves, painful skin injury causes generalized vasoconstriction In addition to the triple response. When body temperature rises during Physical Exercise, cutaneous blood vessels dilate despite ongoing action potentials in noradrenergic synapses elsewhere in the body. The dilation of skin vessels in response to elevated hypothalamic temperature (see Chapter 14) is a powerful reflex response that overrides other reflex activity. Cold causes cutaneous vasoconstriction, but in severe cold, superficial vasodilation may predominate. This vasodilation is the cause of flushed Cheeks in cold weather.

The course of Shock is more severe in febrile patients due to cutaneous vasodilation; such patients should not be warmed to the point where their temperature rises. This is sometimes problematic because well-meaning caregivers know from books that "injury victims should be kept warm," and they cover shock victims with blankets.